Tetrakis(thiadiazole)porphyrazines. 5. Electrochemical and DFT/TDDFT Studies of the Free-base Macrocycle and Its Mg(II), Zn(II) and Cu(II) Complexes

The redox properties of the phthalocyanine-like tetrakis(thiadiazole)porphyrazines, [TTDPzM] (M = MgII(H2O), ZnII, CuII, 2HI), were investigated by cyclic voltammetry, and their ground- and excited-state electronic properties were studied in detail by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. Bulk and specific (axial ligation) solvent effects on the molecular and electronic structure were also taken into account. The title compounds show stepwise reversible ligand-centered one-electron reductions in the range 0 to −2.0 V vs SCE, with E1/2 values being systematically less negative than corresponding reduction potentials for the same processes of the phthalocyanine (Pc) analogues. No electroxidations were observed at positive potentials. The observed redox behavior is rationalized on the basis of the ground-state electronic structures which reveal that replacement of the benzo rings of the Pc macrocycle by electron-withdrawing thiadiazole rings induces a large stabilization of both the HOMO and LUMOs in the investigated macrocycles. An excellent correlation is found between the first one-electron reduction potentials and the gas-phase LUMO energies along the series. The same sequence in the first reduction potentials is theoretically reproduced in pyridine, even if the ZnII and MgII complexes are assumed to be in the axially ligated form, [TTDPzM(py)]. TDDFT calculations of the lowest excited states of the ZnII, MgII, and CuII complexes in pyridine provide an accurate description of their UV−visible spectra. The calculated optical spectra for the free-base macrocycle in chlorobenzene and pyridine confirm previous data in that the thiadiazoleporphyrazine [TTDPzH2] is mostly present in pyridine in its deprotonated form [TTDPz]2-. DFT results, in keeping with electrochemical data, indicate, however, that in pyridine it is the neutral species [TTDPzH2] being reduced instead of its deprotonated form [TTDPz]2-.